To achieve something like that you have to be really strong (this picture is from Atroxia archer BR 624k post-sylph BR increase patch) and of course it also matters how active your server is - the less people the more income for the World Boss participants.
Let's compareThat said, just to put things in perspective let's compare with a few other numbers. I do World Boss on AFK mode with about 75k+ PATK (and 315k BR) and that gets me something like 3 to 3.5 million gold on my server. If I would take control manually then I suppose I could move it up to 4 to 4.5 mil.
The top ranks (1-3) on my server earn about 8-9 million gold and daru.
So clearly 30+ million is exceptional. Oh and also to be more clear here below was the chat message showing that this income is due to 13.49% of damage done on World Boss.